Yoneda lemma
noun
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- (category theory) Given a category \mathcal{C} with an object A, let H be a hom functor represented by A, and let F be any functor (not necessarily representable) from \mathcal{C} to Sets, then there is a natural isomorphism between Nat(H,F), the set of natural transformations from H to F, and the set F(A). (Any natural transformation \alpha from H to F is determined by what \alpha_A(\mbox{id}_A) is.)
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lemma di Yoneda
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